INTERNATIONAL SOUND AND VIBRATION DIGEST Volume 1, Number 2 Date: November 12, 1994 Editor-in-Chief: Malcolm J. Crocker, Auburn University, USA Assistant Editor: Yana Sokolova, Auburn University, USA Editorial Board: Duan-shi Chen, Jiao Tong University, Shanghai, CHINA Jean L. Guyader, INSA de Lyon, FRANCE Colin H. Hansen, University of Adelaide, AUSTRALIA Hanno Heller, DLR, Braunschweig, GERMANY Nikolay Ivanov, Baltic State University, St. Petersburg, RUSSIA Finn Jacobsen, Danish Technical University, DENMARK G. Krishnappa, Institute for Machinery Research, NRC, CANADA Conny Larsson, Uppsala University, SWEDEN Martin V. Lowson, University of Bristol, UK Leonid M. Lyamshev, Andreev Acoustics Institute, Moscow, RUSSIA Eric Marsh, Penn State University, USA M.L. Munjal, Indian Institute of Science, Bangalore, INDIA David E. Newland, The University of Cambridge, UK Michael P. Norton, University of Western Australia, AUSTRALIA A. Selamet, The University of Michigan, Ann Arbor, USA Andrew F. Seybert, University of Kentucky, Lexington, USA Jan W. Verheij, TNO, Delft, THE NETHERLANDS. Current number of subscribers: 1227 To send a submission to the IS&V DIGEST or to subscribe or unsubscribe send information by E-mail to yanas@eng.auburn.edu. TODAY'S DIGEST CONTENTS ITEM 1. Introduction. ITEM 2. OVERVIEW: The Current Status of Sound Intensity Measurements. ITEM 3. PROFESSIONAL SOCIETY: Deutsche Gesellschaft fuer Akustik DEGA. ITEM 4. LABORATORY: Walter E. Lay Automotive Laboratory, University Of Michigan. ITEM 5. The Concert Hall in Nottingham, England and some of its Design Features. ITEM 6. CONFERENCE: ASME 15th Biennial Conference on Mechanical Vibration and Noise. ITEM 7. Session on Structural Acoustics and Anna Pate. ITEM 8. READER QUESTION: Ultrasonic Drivers. ITEM 9. SEMINAR: 20th International Seminar on Modal Analysis (ISMA) Advanced Techniques in Vibro-acoustic Modelling and Analysis. ITEM 10. BOOK REVIEWS: New Books. ITEM 11. CALENDAR of Conferences, Congresses, Meetings & Symposia. ITEM 12. TECHNICAL PAPER: Current Research In Active Control of Noise. ***************************************************************** ITEM 1. INTRODUCTION ***************************************************************** We are very pleased to bring you the second issue of the INTERNATIONAL SOUND AND VIBRATION DIGEST. We should like to thank those authors who have taken time to prepare technical articles and to submit news items of interest to our readers. We should like again to invite readers to submit material for publication. As todayþs DIGEST contents shows, a variety of material is acceptable as long as it is of a high scientific standard and/or the items are newsworthy and of interest to our readers. In this second issue we are pleased to offer several items which we hope will become permanent features of our DIGEST. The first is a series of brief OVERVIEWS of different important topics written by experts. Finn Jacobsen has contributed the first on Sound Intensity (ITEM 2). The second is a series of articles describing the activities of PROFESSIONAL SOCIETIES in different countries which have an interest in the sound and vibration field. Volker Mellert has submitted the first on the Deutsche Gesellscharf fuer Akustik (DEGA) in Germany (ITEM 3). The third is a series of TECHNICAL PAPERS on subjects of current scientific interest. Colin Hansen begins the first of a three part paper on Active Control of Noise and Vibration (ITEM 12). Parts two and three will be continued in future issues of the DIGEST. The fourth item is the first of a series of articles on LABORATORIES involved in sound and vibration research at universities, institutes and companies. In ITEM 4, A. Selamet describes the Walter E. Lay Automotive Laboratory at the University of Michigan. The fifth item is the beginning of a series of BOOK REVIEWS. In ITEM 10 we list brief descriptions of a number of books which have been published recently. Future issues of the DIGEST will carry in- depth reviews of these and other new books in the sound and vibration field. In ITEMS 3, 6, and 9, details are given of a number of conferences and symposia. These are compiled into a CALENDAR OF EVENTS (Conferences, Congresses, Meetings and Symposia) (ITEM 11). The CALENDAR will be presented aperiodically and will also be available on the BULLETIN BOARD run in conjunction with this DIGEST. Last but not least, ITEM 8 contains a question from a reader. We hope that other readers will pose further scientific questions concerning theoretical or measurement problems in the future. We should be very interested to hear your comments concerning the DIGEST and its format. We plan to publish the DIGEST approximately once each month, provided sufficient items have been submitted. Malcolm J. Crocker Editor-in-Chief ***************************************************************** *** ITEM 2. THE CURRENT STATUS OF SOUND INTENSITY MEASUREMENTS. ***************************************************************** *** Equipment for the measurement of sound intensity has been on the market for about 14 years; numerous studies of the sources of error in such measurements have increased our knowledge of the matter; and the first international standards on sound power determination using sound intensity, ISO 9614-1, and on instruments for such measurements, IEC 1043, have been published. The second ISO standard, ISO 9614-2 based on the scanning technique, should be published in less than a year. In addition, many countries have issued national standards, including standards that allow intensity measurements in determining the sound transmission loss of partitions. In other words, the sound intensity technique is now a fairly well established tool. The purpose of the following communication is to give an overview of the current status of sound intensity measurements. The measurement principle based on two closely spaced pressure microphones is totally dominant at the moment. The finite difference approximation error inherent in this measurement principle is not a problem in practice; it simply imposes an upper frequency limit in sound power determination. For example, a microphone separation distance of 12 mm makes it possible to measure with a tolerable accuracy up to about 5 kHz; with 25 mm between the microphones the upper frequency limit is 2.5 kHz. If only low frequencies are of interest it is advantageous to use a large separation distance. By far the most serious source of error associated with this measurement principle is phase mismatch between the two transducer channels. In practice, at the current technical state of the art, this means that the error due to phase mismatch is likely to be the dominating error - unless the measurement takes place in the presence of significant airflow, or the sound power of the source under study is very low. Unless the measurement conditions are extremely difficult, the following simple 'rules' will ensure an accurate result. If the measurement conditions are extremely difficult it will not be possible to satisfy the second 'rule'. i) Use the scanning method, with a distance between parallel lines of less than the distance to the source. (An exception: if the source under study operates in cycles it may be necessary to measure at fixed points; it will usually not be possible to synchronise a scanning measurement to a nonstationary sound field.) ii) Keep an eye on the global pressure-intensity index of the measurement. This quantity, which should be determined concurrently with the actual measurement, should be less than the residual pressure-intensity of the intensity measurement system minus 7 dB. This condition ensures an acceptably small error due to phase mismatch. The sign of the error depends on the sign of the phase error of the measurement system. The residual pressure- intensity index can be determined by exposing the two microphones to the same pressure in a small cavity. iii) Avoid measurements in extreme nearfields of small sources. iv) Measure twice, preferably using another scanning pattern in the second measurement. Watch out for significant deviations between the results; the difference in any band should be a fraction of a decibel. v) Use a windscreen if there is any danger of the intensity probe being exposed to airflow, in particular if the low frequency range is of interest. The most significant improvement of the instrumentation for intensity measurements in the past 10 years has been the development of microphones with reduced production tolerances of the phase characteristics at low frequencies and very low vent sensitivity. The alternative measurement principle, in which a particle velocity transducer and a pressure transducer are combined, has some advantages in comparison with the dominant 'two- microphone' principle, the most important of which is that measurements with such a probe would be far less affected by ambient noise. Unfortunately, calibration and adjustment of the unavoidable phase mismatch between the two transducers is not simple - which means that the 'two-microphone' technique is likely to be dominant also in the future. Finn Jacobsen, The Acousics Laboratory, Technical University of Denmark, dk-2800 Lyngby, Denmark. (fjac@la,dtu.dk) ***************************************************************** *** Finn Jacobsen | E-mail: fjac@la.dtu.dk The Acoustics Laboratory, Building 352 | Telephone Technical University of Denmark. | Direct : +45 4593 1222 ext. 3938 DK-2800 Lyngby | Secretary: +45 4288 1622 Denmark | Fax: +45 4288 0577. ***************************************************************** *** ITEM 3. DEUTSCHE GESELLSCHAFT FUER AKUSTIK DEGA . ***************************************************************** *** Over the last few years there have been some very important changes in the organisation of acoustics and the acousticians in Germany and Europe. The following article will inform you very briefly about these changes. It is believed that the new organisation will lead to a new partnership between acousticians and to new possibilities in scientific communication in all fields of acoustics. The 'Deutsche Arbeitsgemeinschaft fuer Akustik' DAGA, which had the sole aim of organising the DAGA-congress, has completed its work after 23 successful years. As a successor organisation DEGA was founded in 1989. The German Society of Acoustics DEGA is a non-profit scientific association, which was founded in order to integrate the interests of German speaking acousticians, who are working in the fields of applied, technical or scientific acoustics. In the past, different engineering and scientific societies offered specific working groups for acousticians in Germany. But no cohesive interest group of German speaking acousticians existed until the foundation of DEGA. a) Aims of DEGA - to promote acoustics - to organize scientific meetings and congresses, especially the annual DAGA-congress with the participation of several German scientific and engineering societies - to bring together members of the society and everybody else as well, who are interested in acoustics, in order to exchange and gain experience, especially with foreign colleagues - to publish member news and the international journal Acta Acustica - to promote good relations with international societies with similar objectives - to give advise and support activities in research and development, education and professions - to sustain the work of national and international acoustic standardization b) DAGA-Congress The DAGA-congress is the most important congress on acoustics in Germany. The last congress took place in Dresden with more than 800 participants. Announcement for the next DAGA-Congress: DAGA 95 13 - 17 March 1995 IzfP Fraunhofer-Institut fuer zerstoerungsfreie Pruefverfahren Universitaet, Geb. 37 D-66123 Saarbruecken, GERMANY phone: +49 681 302 3836 Fax.: +49 681 30693 c) Organization The main activities of DEGA are performed in working groups with specific scientific and applied fields of interest. Current working groups are concerned with - building and room acoustics - electroacoustics - hearing research - underwater acoustics and geoacoustics - musical acoustics - ultrasound - speech - noise and vibration abatement - physical acoustics - teaching of acoustics The DEGA has an advisory board comprising the chairmen of the working groups and persons elected by the assembly of members. DEGA is represented by the president, the vice-president, the treasurer and three further administrators, at present: President: V. Mellert Vice-president: F.P. Mechel Treasurer: G. Schommartz Further members of the board: J. Blauert, J. Herbertz, A. Lenk d) Membership Anyone interested in acoustics can become a member of DEGA. Students pay a reduced subscription. Furthermore, companies can become sustaining members of DEGA. Members can participate at the annual DAGA--congress at a reduced fee. All personal members will receive the 'Sprachrohr' and the 'Acta Acustica' free of charge. Please contact the DEGA secretariat for further information (address see below). > Publications - A small journal with news for DEGA members is published irregularly 'Sprachrohr'. - 'Acta Acustica' is the journal for DEGA members in which German society news and other information is given. 'Acta Acustica' is the journal of the European Acoustics Association EAA. (There are currently 20 European acoustical societies which are members in this association.) 'Acta Acustica' appears bimonthly and offers the following topics: + Scientific and technical reviewed papers + Book Reviews + Doctoral Thesis Abstracts, prepared by the member societies of EAA. (Abstracts are not reviewed; they may be submitted by any member; please contact the Secretariat) + Society News, prepared by the member societies of EAA + News from Abroad + Congress Reports + Upcoming Events + Deadlines for EC Research and Education Programs + Publication of sound-examples etc. on a Compact Disk - The contributions of the DAGA-congresses are published in the special series 'Fortschritte der Akustik'. Secretariat address DEGA Secretariat Dr. Albert Sill c/o Dept. Physics, Acoustics University of Oldenburg D--26111 Oldenburg Tel.: (+49) 441 / 798-3572 Fax.: (+49) 441 / 798-3698 E--mail: DEGA@aku.physik.uni-oldenburg.de Prof. Dr. Volker Mellert President of DEGA ***************************************************************** *** ITEM 4. WALTER E. LAY AUTOMOTIVE LABORATORY, UNIVERSITY OF MICHIGAN. ***************************************************************** *** In recent years an extensive analytical, computational, and experimental research effort has been undertaken at the Walter E. Lay Automotive Laboratory of the University of Michigan. The objective of the research is to develop and validate computational models in the time-domain for the design of vehicle induction and exhaust systems for minimum noise, power loss, and emissions. To achieve this objective, a number of facilities have been designed and instrumented. These are described in the following: 1. Extended Impedance Tube - Acoustics Laboratory: The extended impedance tube acoustics test facility has been designed to determine experimentally the acoustic wave suppression primarily for the validation of computational simulations. This facility measures the acoustic characteristics of both fundamental silencers and vehicle production hardware by implementing the two microphone technique. Four microphone signals from two pairs mounted upstream and downstream of the acoustic elements are processed by a state-of-the-art modular, multichannel analysis system (Bruel and Kjaer 3550), which includes a signal analyzer (B&K 2035) and a multichannel data acquisition unit (B&K 2816) coupled with a compatible 100kHz-zoom processor, 25kHz-input, and interface modules. To date, fundamental silencers, including expansion chambers, Helmholtz resonators, quarter-wave side-branch resonators, Herschel-Quincke and Herschel-Venturi tubes, and perforated resonators, have been computationally simulated and the results have been validated in this laboratory. Extensive studies are currently underway to build and validate models for more complicated production hardware. 2. Engine Breathing System - Wave Dynamics Laboratory: The laboratory is designed to understand the fundamental behavior of vehicle induction and exhaust system components, particularly catalytic converters, resonators, and mufflers, and thereby validate the computational models being developed to predict engine performance and noise suppression. The ultimate objective is to guide the practical design of these elements and the overall breathing systems with the emphasis on: (1) smaller packaging volumes; (2) lower flow losses for minimal engine horse- power loss; (3) improved noise reduction; and (4) reduced cost. The experiments are conducted by using a firing (or motored) 3.0L V6 Vulcan Spark-Ignition engine with full vehicle induction and exhaust systems. Measurements include the time-averaged quantities, such as torque, power, air and fuel flow rates, surface and gas temperatures, and the time-resolved quantities, including the absolute dynamic pressures of the induction air and the hot exhaust gases at key locations (upstream and downstream of every component) with Kistler fast-response, (water-cooled for exhaust side) piezo-resistive transducers. The nonlinear acoustic performance of each component as well as the flow losses caused by these elements and their influence on the engine performance are then determined and compared with the model predictions. The facility includes a computer-controlled cooling stand for engine cooling water and oil, and a Concurrent SLS5450-01 Scientific Lab high-speed data acquisition system, which is also connected to HP 9000/735/125 workstations through the network. 3. Advanced Spark Ignition Engine Exhaust Systems and Pollutant Emissions Laboratory: In addition to studying the fundamental wave dynamics and heat transfer characteristics of vehicle induction and exhaust systems, this laboratory provides emissions equipment to measure pollutants in the exhaust system. The emission benches built by Horiba combined with the EDTCS 1000 computer-controlled dynamometer system facilitates the acquisition of all time-averaged quantities, including, for example, torque, power, air (through Meriam flow meter) and fuel flow rates (through Pierburg/Flotron flow meter); ambient, cooling water and oil, surface and gas temperatures; ambient, intake air, oil and fuel pressures; and pollutant emissions such as carbon monoxide, carbon dioxide, hydrocarbons, and nitrogen oxides. The wave dynamics in the induction and exhaust systems and the other time-resolved quantities are captured by a state-of-the-art, Concurrent 7250-1C70 high speed data acquisition system, which is also connected to HP 9000/735/125 workstations through the network. In this unique facility, the performance, wave dynamics and noise attenuation, and emissions in spark-ignited engines - currently Ford 1.9L I4 Escort engine - are investigated simultaneously for the development of (1) computational models; and (2) novel breathing system elements. The study is expected to minimize preliminary fabrication and testing in vehicle development, thereby reducing the design cycle of a new vehicle. A. Selamet Department of Mechanical Engineering and Applied Mechanics Department 120 W. E. Lay Automotive Laboratory The University of Michigan Ann Arbor, MI 48109-2121 selamet@um.cc.umich.edu or sel@engin.umich.edu ***************************************************************** *** ITEM 5. THE CONCERT HALL IN NOTTINGHAM, ENGLAND AND SOME OF ITS DESIGN FEATURES. ***************************************************************** *** Royal Concert Hall, Nottingham Seat Count: 2500 Background Noise Level: PNC-15 Designed: 1979-81 Opened: 1982 Built For: Nottingham City Council, Chief Executive in 1982: Roger Haslam In 1979 the City of Nottingham, England (population about 250,000) decided to build a concert hall adjacent to the nineteenth century 1100-seat Theatre Royal. The program for the new hall was to be 50% symphony music and 50% popular music. Clearly, a hall with a single fixed acoustic would not meet the requirements. The Royal Concert Hall was the first concert hall in the UK to be built with adjustable acoustics -- as distinct from acoustics that could be "tuned" if the hall did not turn out as intended. The acoustical adjustment features include an overhead canopy that raises/lowers and tilts local reflectors behind the musicians (visually concealed by the faces of the chorus risers) and sound- absorbing acoustical banners that retract into storage pockets in the ceiling. The acoustical design was constrained by planning restrictions governing the height of the building and by the boundaries of the site which run within a few metres of the auditorium on all sides of the building. Within the volume available, our acoustical design was developed to provide strong early reflections in the seating areas, and a powerful foundation for the sound of the lower strings. This was achieved by placing reflective surfaces to provide the early reflections, and by using large flat surface areas (typically 12' x 18') with massive materials (5" concrete) to maintain the strength of the low-frequency reverberant sound. The audience layout, seating and sightlines were, to a large extent, determined by the theatre planners (Theatre Projects) and the architects (RHWLP). However, we influenced the shaping of the room by introducing important parallel wall surfaces near the platform and by carefully arranging the balcony overhangs at the rear to minimize acoustic shadowing. The only place we could position the reflectors was in the upper part of the room -- the upper side walls and the ceiling. We used mirrored surfaces to model the angles of the ceiling planes using a light source. We were designing (in those days) using first-order reflections, which lead to strongly tilted and angled ceiling planes, typical of the "directed sound energy" concert hall shaping. With so large a seat count, the design of the seating tiers and overhangs is particularly critical; the small overhang in the stalls gives way to deeper overhangs at the higher levels. The seats in the stalls area are served by multiple reflections from the rear wall/front wall and between the parallel side walls. These seats, and those for the musicians on the platform, are served by reflections from the large single-piece vertically adjustable acoustic canopy. One of our concerns with the canopy design was that the steel structure of the canopy would "ring" due to lack of damping. We were advised to sandwich neoprene around the steel tubes at the fixing points, using load-spreading plates. This has worked, and the structure does not ring. We have had problems (on other canopies) with steel conduits causing the canopy to "hum" (electromagnetic effects from the dimmed power to the concert/house lights inducing vibration in the conduit) which confirms the high radiation efficiency of the canopy. The neoprene works for three reasons: 1) it reduces the transmission of vibrational energy from the canopy surface into the steel, 2) it reduces vibrational transmission from the steel back into the canopy, and 3) it damps vibration of the steel tubes. The Nottingham hall opened in 1981, and is a great success. At the opening CBSO concert, the young conductor Simon Rattle met me (Nicholas Edwards) and Russ Johnson and told us "I'm going to have you design a new concert hall in Birmingham". We did indeed design one (Symphony Hall, Birmingham) in 1985, and I moved back to the UK to be on the site in 1989. After the hall, Birmingham Hall, opened in 1991 I joined with David Kahn, Craig Janssen and others to start our own acoustics consulting firm "Acoustic Dimensions", with offices in New York, Dallas and Coventry, UK. Nicholas Edwards 100114.722@compuserve.com ***************************************************************** *** ITEM 6. ASME 15TH BIENNIAL CONFERENCE ON MECHANICAL VIBRATION AND NOISE. ***************************************************************** *** Call For Papers ASME 15th Biennial Conference on Mechanical Vibration and Noise Boston, Massachusetts, September 17-21, 1995 Symposium on Transient Signal Processing and Wavelets in Vibrations and Acoustics Suggested topics include but are not limited to: Application of wavelets to vibration and sound Time-frequency analysis Hilbert transform Covariance methods Envelope or phase-based processing Machine diagnostics Transient wave propagation Inverse problems and system identification Measurement techniques or signal processing algorithms Impact and shock response Isolation for transient response Important Dates: Abstract Deadline: December 15,1994 Manuscripts (Four Copies) January 15, 1995 Notification of Acceptance April 1, 1995 Final paper on Mats April 25, 1995 Submit a one-page abstract by December 15, 1994 to either of the symposium organizers: Professor Andres Soom Mechanical and Aerospace Engineering Department State University of New York at Buffalo 321 Jarvis Hall, Buffalo, NY 14260-4400 Phone: (716) 645-2593, ext. 2236, Fax: (716) 645- 3875 E-mail: mecsoom@ubvms.cc.buffalo.edu Papers dealing with wavelet-based methods should be sent to: Professor David E. Newland Department of Engineering, University of Cambridge Trumpington Street, Cambridge CB2 1PZ Phone: 44 223 33267, Fax: 44 223 359153 E-mail: den@eng.cam.ac.uk ***************************************************************** *** ITEM 7. SESSION ON STRUCTURAL ACOUSTICS AND ANNA PATE. ***************************************************************** *** Professor Anna L. Pate, a faculty member in the Aerospace Engineering and Engineering Mechanics Department at Iowa State University, died of cancer in Jan. 1993 at age 44. Anna received a Master's degree in Electrical Engineering from the Technical University of Warsaw, Poland in 1972 and the Ph.D. in Mechanical Engineering from the Technical University of Krakow, Poland in 1978. From 1973 until 1978 she was an instructor in the Institute of Mechanics and Vibroacoustics at the Technical University of Krakow, and after a postdoctoral research appointment at the Ray W. Herrick Laboratories at Purdue University she returned to Krakow as an assistant professor in the Institute of Mechanics and Vibroacoustics. Anna returned to the United States in 1980, and was a consultant in acoustics and noise control in San Francisco and graduate instructor in the Department of Mechanical Engineering at the University of Santa Clara until 1982 when she joined the faculty of Iowa State University. She served as Assistant Dean of Engineering for Faculty Development from January 1989 through June 1990. Anna's principal areas of research and teaching were acoustics, vibrations and noise control. She was an international authority in structural acoustics, and was a respected teacher and mentor. Anna's research accomplishments during her brief career were prodigious. She supervised the research of 7 MS and 7 PhD students, most of whom she supported in grants that totaled over $2,200,000 since 1984; she was the author or co-author of more than 30 publications; she gave more than 50 technical presentations at professional meetings, universities, and for industry in the U.S. and other countries; and she was a sought after session organizer and reviewer for papers and proposals. Her work in sound intensity, acoustic holography and structural acoustics was well known and respected by colleagues around the world. Anna's professional success also led to increased professional and university service activity. She was a member of the Editorial Board of the Noise Control Engineering Journal, a member of the Executive Committee of the ASME Division of Noise Control and Acoustics and was the Chair of the Technical Committee on Experimental Acoustics and Instrumentation of ASME and also served on the Instrumentation Group of the Structural Acoustics Technical Committee of the Acoustical Society of America. At Iowa State she served as Chair of the Physical Review Committee of the Graduate College, as a member of the Women in Science and Engineering Committee, the NDE Educational Committee and numerous other college, department and university committees. Anna was always willing to give of her time and abilities to further education, research or the development of the students, faculty and colleagues around her. Her personal contributions to the department, university, and her profession were outstanding. In honor of her memory, the Acoustical Society of America technical committees on Structural Acoustics and the Status of Women plan to hold a special session: "A Celebration of the Life and Contributions of Anna L. Pate" at the fall ASA 1995 meeting to be held in Washington DC. Persons wishing to present technical work in areas related to Anna's interests at this session may submit a brief abstract to session co-chairs David Holger and Alison Flatau at abf@iastate.edu, or at Iowa State University, 2019 Black Engr. Bldg., Ames, IA 50011. ***************************************************************** *** ITEM 8. ULTRASONIC DRIVERS. ***************************************************************** *** I am looking for sources of ultrasonic speakers to have a relatively flat frequency response in the range of 30-100 kHz. We have been using a leaf tweeter that is out of production. These are used in experiments to study the biosonar system of echolocating bats. Thanks for any information. David M. Gooler University of Illinois Beckman Institute - NPA Group 405 N. Mathews Avenue Urbana, IL 61801 Telephone: (217) 333-7071 FAX: (217) 244-5180 EMAIL: dgooler@synapse.npa.uiuc.edu ***************************************************************** *** ITEM 9. 20TH INTERNATIONAL SEMINAR ON MODAL ANALYSIS (ISMA) ADVANCED TECHNIQUES IN VIBRO-ACOUSTIC MODELLING AND ANALYSIS. ***************************************************************** *** First announcement of the 20th International Seminar on Modal Analysis (ISMA) Advanced Techniques in Vibro-acoustic Modelling and Analysis to be held in Leuven, Belgium, 11-13 September 1995. The Mechanical Engineering Department of the Katholieke Universiteit Leuven, Belgium will celebrate the 20th edition of its traditional ISMA Seminars next year. Two parallel courses will be organized: one on structural modal analysis, and the other on advanced methods in applied and numerical acoustics, this time mainly devoted to vibro-acoustic modelling and analysis methods. The main topics of the acoustics course are as follows: - Analysis tools for subjective noise evaluation - Noise and vibration Transfer Path Analysis - Statistical Energy Analysis - Acoustic and vibro-acoustic modal analysis - Acoustic Finite Element and Boundary Element Analysis - Medium-frequency and non-modal modelling techniques - New descriptors of acoustic sources with respect to vibro- acoustic modelling The various topics will be serveyed in theoretical lectures and illustrated by subsequent demonstrations on a simple but versatile laboratory measurement set-up. The seminar is application-oriented with emphasis on the background and principles but mainly on the practical use of the methods discussed, with special regard to the automotive and aerospace industries. The seminar is intended for researchers and engineers in the field of noise and vibration analysis, who wish to update their knowledge on some recent topics in vibroacoustics. For more information on the 20th ISMA or inquiries about proceedings of earlier seminars please contact Mrs. L. Notre Dept. of Mechanical Engineering Katholieke Universiteit Leuven Celestijnenlaan 300 B B-3001 Heverlee, Belgium Tel: .Int + 32 16 28 66 11,(from 1 January 1995: + 32 16 32 24 80)... Fax: .Int + 32 16 22 23 45,(from 1 January 1995: + 32 16 32 29 87) E-mail: Lieve.Notre@mech.kuleuven.ac.be And for those who have never been to Leuven (by the way, the venue of Inter-Noise 93), it is worth mentioning here that it is a town of medieval origins, located in the heart of Belgium, only 30 km from Brussels and 20 km from the Brussels National Airport. The historic and artistic past is present everywhere in this charming city which is the true capital of Brabant. Brussels may be the first, but Leuven is undoubtedly the loveliest. And do not forget about the special evening sessions of the seminar in one of the numerous beer cellars around the famous Town Hall either... ***************************************************************** *** ITEM 10. NEW BOOKS. ***************************************************************** *** ENVIRONMENTAL AND ARCHITECTURAL ACOUSTICS, Z. Maekawa and P. Lord, E. and FN Spon, Chapman, and Hall, London, 1994, xi + 377 pp. This book represents the life work of the first author. It contains 10 chapters covering fundamentals of sound waves, room acoustics, sound absorption and transmission. Noise and vibration measurement, rating and their control are also discussed. In addition the acoustic design of rooms and electro-acoustic systems are described. The book appears most useful for architectural students. Problems are given at the end of each chapter, but no worked solutions or answers in this volume. _________________________________________________________________ ___ INDUSTRIAL NOISE CONTROL by Paul N. Cheremisinoff, PTR Prentice Hall, Englewood Cliffs, NJ, 1993, ix + 192 pp., $66.00. This is rather a short book. It is mostly descriptive and designed for practitioners. It covers all the main topics one would expect in 12 chapters and a glossary: noise effects and regulations, acoustics, engineering controls and design, personal safety (ear protection), enclosures and barriers, vibration and its control, ventilating and interpolation and mapping. _________________________________________________________________ ___ INDUSTRIAL NOISE CONTROL: FUNDAMENTALS AND APPLICATIONS Second Edition, by Lewis H. Bell and Douglas H. Bell, Marcel Dekker, Inc., New York, 1993, xiv + 660 pp, $150.00. This practical book is divided into four main parts: (I) Nature and Measurement of Sound, (II) Noise Control Methods, (III) Basic Sources of Noise, (IV) Environmental Acoustics. There is a total of 18 chapters and 9 Appendices. Part III contains descriptive treatments of the noise of fans, gas jets, gears, punch presses, hydraulic pumps, electrical equipment and some other industrial equipment. _________________________________________________________________ ___VIBRATIONS OF SHELLS AND PLATES Second Edition by Werner Soedel, Marcel Dekker, New York, 1993, xix + 470pp., $135.00. This book is designed mainly for graduate students and practicing engineers. It is a survey of shell and plate vibration theory in 21 chapters. The author is an accomplished theoretician, but his practical experience and physical insights are evident throughout the book. Each chapter is accompanied by useful lists of references, but surprisingly for a book of this type, no questions or answers are provided. Malcolm J. Crocker ***************************************************************** *** ITEM 11. CALENDAR OF CONFERENCES, CONGRESSES, MEETINGS & SYMPOSIA . ***************************************************************** *** IOA Autumn Conference: November 17-20, 1994, Windermere, UK. Sessions on noise are planned. Contact: C.M. Mackenzie, Institute of Acoustics, P.O. Box 320, St. Albans, Herts AL1 1PZ, UK. Tel: +44 727 848195, Fax: +44 727 850553. 128th Meeting of the Acoustical Society of America: November 28- December 2, 1994, Austin, Texas, USA. Contact: Elaine Moran, Acoustical Society of America, 500 Sunnyside Blvd., Woodbury, NY 11797, USA. Tel: +1 516 576-2360, Fax: +1 516 349-7699. Euronoise `95: Software for Noise Control, March 21-23, 1995, Lyon, FRANCE. Contact: CETIM Acoustical Department, B.P. 67, 60304 Senlis, FRANCE. Tel: +33 4 458 3217, Fax: +33 4 458 3400. Hearing Conversation Conference III/XX: March 22-25, 1995, Cincinnati, Ohio, USA. Sessions on noise cancellation and control techniques are planned. Contact: Michele Johnson, National Hearing Conversation Association, 431 East Locust Street, Suite 202, Des Moines, IA 50309, USA. Tel: +1 515 243 1558, Fax: +1 515 243 2049. International Conference on Computational Acoustics: April 5-7, 1995, Environmental Applications, Southampton, UK. Contact: J. Evans, Conference Secretariat, Wessex Institute of Technology, Ashurst Lodge, Ashurst, Southampton S04 2AA, UK. Tel: +44 703 293223, Fax: +44 703 292853. Vibration and Noise '95: April 25-27, 1995, Venice, ITALY. Contact: M.J. Goodwin, School of Engineering, Staffordshire University, P.O. Box 333, Beaconside, Stafford ST18 0DF, ENGLAND. Tel: +44 785 275242, Fax: +44 785 227741. International Symposium in Music and Concert Hall Acoustics (MCHA95): May 15 to 18, 1995, Kirishima, Kagoshima-Prefecture, JAPAN. Contact: Yoichi Ando, Faculty of Engineering, Kobe University, Rokkodai, Nada 657 Kobe, Japan. FAX: +81-78-881-3921. E-mail: andoy@icluna.kobe-u.ac.jp SAE Noise and Vibration Conference: May 15-18, 1995, Traverse City, Michigan, USA. Contact: Mone Asensio, SAE International, 3001 West Big Beaver Road, Troy, Michigan, USA. Tel: +1 313 649 0420. 2nd International Conference on Acoustics and Musical Research: 3rd week, May 1995, Ferrara, ITALY. Contact: Conference Secretariat, CIARM95, National Research Council of Italy, Cemoter Acoustics Department, Via Canal Bianco, 28-44044 Ferrara. Tel: +39 532 731571, Fax: +39 532 732250. E-mail: CIARM95@CNRFE4.FE.CNR.IT 129th Meeting of the Acoustical Society of America: May 31-June 4, 1995, Washington, DC, USA. Contact: Elaine Moran, Acoustical Society of America, 500 Sunnyside Blvd., Woodbury, NY 11797, USA. Tel: +1 (516) 576-2360, Fax: +1 (516) 349-7669. 1995 AIAA Aeroacoustics Conference: June 12-15, 1995, Forum der Technik/Deutsches Museum, Munchen, GERMANY. Contact: Dr. Hanno Heller, Abteilung Technische Akustik, DLR-Instotute fur Entwurfsaerodynamik, Lilienthalplatz 7, D-38108, Braunschweig, GERMANY, Tel: +49-5-31-2-95-21-70, FAX: +49-5-31-2-95-2320. Noise Control'95: June 20-22, 1995, Central Institute for Labor Protection, Warsaw, POLAND. Contact: D. Koradecka, Central Institute for Labor Protection, Czernikowska 16, 00-701 Warsaw, Poland. Tel: +482-623-36-78, +482-623-36-60, FAX: +482-623-36-95. 15th International Congress on Acoustics: 26-30 June, 1995, Trondheim, NORWAY. Contact: ICA'95, SEVU, Congress Department, N- 7034 Trondheim, Norway, Tel: +47 7359 5251/7359 5254, Fax: +47 7359 5150, E-mail: ica95@sevu.unit.no. International Symposium on Musical Acoustics ISMA'95: July 2-5, 1995, Dourdan, FRANCE. Contact: ISMA'95 Secretariat, c/o/ Rene Causse, IRCAM, 1 Place Igor Stravinsky, 75004 Paris, FRANCE. Tel: +33-1-44-78-47-60, FAX: +33-1-42-77-29-47, E-mail: isma@ircam.fr. International Symposium on Active Control of Sound and Vibration: July 6-8, 1995. Continuation of two conferences, one organized by Virginia Polytechnic Institute and another by Acoustical Society of Japan. 1995 Symposium sponsored by ASA, ASJ, INCE/USA and INCE/Japan. Contact: J. Tichy, Applied Research Laboratory, Penn. State University, University Park, PA 16802, USA. Tel: +1 814 865 6364, Fax: +1 814 865 3119. INTER-NOISE 95: July 10-12, 1995, Newport Beach, California, USA. Contact: Institute of Noise Control Engineering, P.O. Box 3206, Arlington Branch, Poughkeepsie, NY 12603, USA. Tel. +1 (914) 462- 4006, Fax. +1 (914) 473-9325. 17th Boundary Element International Conference: 17-19 July, 1995, Wisconsin, USA. Contact: Lis Johnstone, Conference Secretariat, BEM 17, Wessex Institute of Technology, Ashurst Lodge, Ashurst Southampton, SO407AA. Tel: 44 (0) 703293223, Fax: 44 (0) 703 292853, E-Mail: CMI@uk.ac.rl.ib, Intl E-Mail: CMI@ib.rl.ac.uk. Second International Conference on Theoretical & Computational Acoustics: August 21-25, 1995, Hawaii, USA. Contact: Dr. Ding Lee (Code 3122), Naval Undersea Warfare Center, Detachment New London, New London, CT 06320, USA. Tel: +1 (203) 440-4438, Fax: +1 (203) 440-6228. 1995 World Congress on Ultrasonics: September 3 to 7, 1995, Berlin, GERMANY. Contact: WCU'95 Secretariat, Prof. Dr. J. Herbertz, Gerhard-Mercator-Universitat, D-47048 Duisburg, Germany. Tel: +49 (203) 379-3243, Fax: +49 (203) 379-3534. BETECH 95: September 13-15 1995, Liege, BELGIUM. Contact: Liz Johnstone, Conference Secretariat-BETECH 95, Ashurst Lodge, Ashurst, SO40 7AA UK. Tel +44 (0) 703 293223, Fax +44 (0) 703 292853, E-Mail CMI@uk.ac.rl.ib., Intl E-Mail CMI@ib.rl.ac.uk. ASME 15th Biennial Conference on Mechanical Vibration and Noise Boston, September 17-21, 1995, Massachusetts, USA. Contact: Professor Andres Soom, Mechanical and Aerospace Engineering Department, State University of New York at Buffalo, 321 Jarvis Hall, Buffalo, NY 14260-4400. Tel: (716) 645-2593, ext. 2236, Fax: (716) 645-3875, E-mail: mecsoom@ubvms.cc.buffalo.edu. Acoustical and Vibratory Surveillance Methods and Diagnostic Techniques: October 10-12, 1995, Paris-Clamart, France. Contact: Jean Fabri, Societe Francaise des Mecaniciens, 39-41 rue Louis Blanc, F-92400 Courbevoie, FRANCE. Tel: 33-1-47-17-64-89, FAX: 33- 1-47-17-61-31. 130th Meeting of the Acoustical Society of America: November 27- December 1, 1995, St. Louis, Missouri, USA. Contact: Elaine Moran, Acoustical Society of America, 500 Sunnyside Blvd., Woodbury, NY 11797, USA. Tel: +1 (516) 576-2360, Fax: +1 (516) 349-7669. Forum Acusticum: April 1-4, 1996, European Acoustics Association, Antwerp, BELGIUM. Contact: A. Cops, Catholic University Leuven, Celestijnenlaan 200D, B-3001 Leuven-Heverlee, BELGIUM. Tel: +32 16 201015, Fax: +32 16 201368. 131st Meeting of the Acoustical Society of America: May 13-17, 1996, Indianapolis, Indiana, USA. Contact: Elaine Moran, Acoustical Society of America, 500 Sunnyside Blvd., Woodbury, NY 11797, USA. Tel: +1 516 576 2360, Fax: +1 516 349 7669. INTER-NOISE 96: July 31-August 2, 1996, The 1996 International Congress on Noise Control Engineering, Liverpool, ENGLAND. Contact: C.M. Mackenzie, Institute of Acoustics, P.O. Box 320, St. Albans, Herts, AL1 1PZ, UK. Tel: +1 44 727 848195, Fax: +1 44 727 850553. 132nd Meeting of the Acoustical Society of America: December 2-6, 1996, Honolulu, Hawaii, USA. Contact: Elaine Moran, Acoustical Society of America, 500 Sunnyside Blvd., Woodbury, NY 11797, USA. Tel: +1 516 576 2360, Fax: +1 516 349 7669. ***************************************************************** *** ITEM 12. CURRENT RESEARCH IN ACTIVE CONTROL OF NOISE. ***************************************************************** *** Colin H. Hansen Department of Mechanical Engineering University of Adelaide South Australia 5005 E-mail: CHANSEN@edison.aelmg.adelaide.edu.au 1.0 INTRODUCTION Since the original idea was conceived in the 1930s [1,2], the active control of sound as a technology has been characterised by transition: transition from a dream to practical implementation and from a laboratory experiment to mass production. This transition has taken a long time, partly because of the time it took to develop sufficiently powerful signal processing electronics, partly because of a lack of understanding of the physical principles involved and partly because of the multi-disciplinary nature of the technology which combines a wide range of technical disciplines including Physics, Electrical Engineering, Materials Science and Mechanical Engineering. Being a collection of pieces, in which the strength of the chain is only as strong as its weakest link, it is little wonder that the technology has been characterised by advances which have come in a series of spurts rather than in a continuous flow. To put current research in perspective, it is useful to briefly review some of the important past milestones. After the exposition of the original idea of active control of noise in ducts in the 1930s, it was not until the 1950s that the idea was rekindled, this time by a man named Olson [3,4] who investigated possibilities for active sound cancellation in rooms, in ducts and in headsets and earmuffs. Again limitations in the available electronic control hardware as well as limitations in control theory prevented this technology from being commercially realised. In the late 1970s and 1980s there was a resurgence of interest in active sound cancellation. Advances in control theory and perhaps more importantly, advances in micro-electronics meant that commercial systems were technically achievable. The result was the installation of a number of "prototype" systems in industrial facilities to control low frequency noise in situations where existing passive control techniques were exorbitantly expensive or impractical. In spite of these advances, there were still a number of technical problems which prohibited widespread implementation of the technology. Perhaps the most limiting of these was associated mainly with the availability of transducers and actuators; stable, high power, low-frequency-response sound and vibration sources and rugged sensors capable of continuous operation for long periods of time in harsh industrial environments were simply not available. Other factors which slowed the development of commercial active control systems in the 1980s include: insufficient experience with practical installations; complexity and cost of systems; lack of education of designers and potential users; insufficient evidence of cost savings, long term performance and reliability; and lack of sufficient marketing effort. In the 1990s, many of these problems will be overcome; inexpensive multi-channel electronic controllers and inexpensive, robust transducers and actuators are being developed. "Chipsets" aimed specifically at mass market implementations of active sound and vibration control exist in the prototype stage and are being aggressively marketed by their manufacturers. Research in active sound and vibration control is also expanding: the number of technical papers published on the topic since Lueg's work in the 1930s is increasing exponentially, from approximately 240 before 1970 to 850 in the 1970s and to 2200 in the 1980s, a trend which is continuing in the 1990s. As integrated microprocessors dedicated to signal processing become cheaper and faster (the speed having doubled every 18 months for the last 10 years), potential active control applications increase in number. However, it should not be assumed that more processing power will extend the applications endlessly. There are some supposedly potential applications (for example, control of traffic noise in living rooms) which will remain impractical, no matter how much processing power is available because the limitations are a result of the structural/acoustic characteristics of the problem. Although more powerful signal processing electronics help to alleviate the electronic problems associated with extending the application of active control to higher frequencies and to more complex multi-channel problems, the structural / acoustic limitations mentioned remain. For the example cited above, to provide significant (or any) attenuation of the unwanted disturbance, a vast array of sensors and actuators would be required: it would be cheaper to build a thicker wall!. The efficiency of active noise and vibration control systems depends upon the design of and harmony of operation between two major subsystems: the "physical" system, and the electronic control system. The physical system encompasses the required transducers: the "control sources" for inducing the secondary disturbance, and the "error sensors" which monitor the performance of the active control system by providing some measure of the residual noise and/or vibration field. Thus, the physical system provides the structural/acoustic interface for the active control systems, and the electronic control system drives the physical system in such a way that the unwanted primary source noise and/or vibration field is attenuated. The quality of the design of these two major subsystems is the critical factor in determining the ability of the active control system to produce the desired results. The design of the physical system, comprising the arrangement of control sources and error sensors, limits the maximum noise or vibration control that can be achieved by an ideal active controller. The control electronics limit the ability of the active control system to reach this maximum achievable result. Thus, although the influence of the quality (or lack thereof) of the two major subsystems manifest themselves in different ways, no active control system can function efficiently with an inefficient physical or electronic subsystem. 2.0. APPLICATIONS OF ACTIVE CONTROL Some typical applications for the application of active noise control which are currently being investigated by research groups throughout the world are listed below (no doubt this is not a complete list). 1. Control of aircraft interior noise by use of lightweight vibration sources on the fuselage and acoustic sources inside the fuselage. 2. Reduction of helicopter cabin noise by active vibration isolation of the rotor and gearbox from the cabin. 3. Reduction of noise radiated by ships and submarines by active vibration isolation of interior mounted machinery (using active elements in parallel with passive elements) and active reduction of vibratory power transmission along the hull, using vibration actuators on the hull. 4. Reduction of internal combustion engine exhaust noise by use of acoustic control sources at the exhaust outlet or by use of high intensity acoustic sources mounted on the exhaust pipe and radiating into the pipe at some distance from the exhaust outlet. 5. Reduction of low frequency noise radiated by industrial noise sources such as vacuum pumps, forced air blowers, cooling towers and gas turbine exhausts, by use of acoustic control sources. 6. Lightweight machinery enclosures with active control for low frequency noise reduction. 7. Control of tonal noise radiated by turbo-machinery (including aircraft engines). 8. Reduction of low frequency noise propagating in air conditioning systems by use of acoustic sources radiating into the duct airway. 9. Reduction of electrical transformer noise either by using a secondary, perforated lightweight skin surrounding the transformer and driven by vibration sources or by attaching vibration sources directly to the transformer tank. Use of acoustic control sources for this purpose is also being investigated, but a large number of sources are required to obtain global control. 10. Reduction of noise inside automobiles using acoustic sources inside the cabin and lightweight vibration actuators on the body panels. 11. Active headsets and earmuffs. 3.0 CURRENT RESEARCH Current research may be divided into two main categories: electronic control system hardware and software development, and physical system design (including transducer design and location). Even though practical systems have been installed in industry (mainly in air handling ducts and in active head sets), there is a large on-going research effort directed at the development and implementation of more complex systems. Most existing successful applications are single channel systems controlling either plane waves in a wave guide or long wavelength sound fields in small cavities (head sets). The challenge which is in the process of being met is to extend the application of the technology to higher frequencies and to multi-channel systems (multiple control sources and error sensors) for the control of more complex acoustic environments. A large proportion of the current research in the USA is being undertaken by a few commercial companies (Digisonix, Noise Cancellation Technologies and SRI International), in the Mechanical Engineering Departments of a number of Universities (Virginia Polytechnic Institute and State University, Pennsylvania State University, Purdue University and Duke University) and at NASA Langley. In the UK, the principal research establishments are ISVR at the University of Southampton and the Department of Automatic Control and Systems Engineering at the University of Sheffield. In Japan, there is a large group working on active control at the Mechanical Engineering Laboratory of the Ministry of Trade and Industry at Tsuba, as well as considerable commercial activity at Nissan and Mitsubishi. In Australia research in active control is being undertaken at the University of Adelaide, Department of Mechanical Engineering and by the commercial company, Causal Systems. Other well known groups are in Italy (CEMOTER -National Research Council), Germany (University of Goettingen), France (Centre for National Scientific Research), Denmark, Netherlands (Institute of Applied Physics), Belgium (University of Ghent, Department of Information Technology), China and Korea. In the remainder of this article, areas of current research will be identified and briefly described. Much of the current research activity in active noise control is reported in the Proceedings of specialist conferences (for example; Noise Con '94; Internoise '94; the Second and Third International Congresses on Air and Structure - borne Sound and Vibration, 1992 and 1994; Acoustical Society of America, May and September meetings each year; and the Congress on Recent Advances in Active Control of Noise and Vibration, 1991, 1993 and 1995). 3.1 Control system hardware Work on control system hardware is currently directed at making use of state-of-the-art digital signal processors (DSPs) to design compact multi-channel systems which have acceptable cost/performance ratios and which have as small a signal throughput delay as is possible. Smaller throughput delays result in more compact physical systems so that the reference signal sensor and control source may be placed closer together without violating causality constraints. Unfortunately, low cost analog to digital converters with inbuilt anti-aliasing filters are characterised by unacceptable delays (typically 32 samples compared to about 5 s for a standard A/D converter) when low sampling rates, suitable for active noise and vibration control are used. One way around this problem is to sample at a high rate, use a low pass digital filter implemented in software and then throw away nine out of every ten filtered samples. For systems with floating point processors an IIR filter is an appropriate digital filter implementation, but for fixed point processors a modification is required to make the filter work properly in practice because the fixed point processor numerical range is limited to unity and IIR filters require larger numbers for their weight coefficient values. Alternative filter structures may be able to overcome this problem but for now the solution is to use a floating point software implementation. However this is computationally expensive and one way around this is to use block floating point specification of the weights in each of the two FIR filters making up the IIR filter. Research is currently in progress aimed at producing hardware development systems consisting of a printed circuit board, a DSP, multi-channel software and all of the necessary electronic components so that consumer product manufacturers and researchers can use it to optimise active control system designs with the view of eventually producing a low cost system embedded in their product (an example of a development system is the EZ-ANC system offered by the Australian company, Causal Systems). Because consumer product systems must be as inexpensive as possible, fixed point processor hardware is preferred because it is always less expensive than floating point hardware. However fixed point hardware has a number of limitations [5] as a result of the limited precision definition of numbers such as adaptive filter weights and convergence coefficients and great care must be taken to ensure that the resulting system is robust and has the required performance. Where final product cost is not too critical, floating point systems are generally preferred. 3.2 Control system software This is an area of a large amount of research activity with effort being spent on new filter architectures and algorithms aimed at reducing adaptation times and increasing system performance and robustness. The cancellation path transfer function in feedforward systems is continuing to attract a significant amount of attention with suggestions of improved estimation schemes [6] and other algorithms which eliminate the need to determine it [7-9]. One of these algorithms, the genetic algorithm, will be discussed in more detail later. Faster adaptive feedforward algorithms which are more effective for random noise and more robust than traditional algorithms are also the subject of considerable research effort [10] as is the implementation of more efficient and effective filter structures [11]. Work is also continuing on the development of frequency domain algorithms which include a frequency domain convergence coefficient, allowing different emphasis to be placed on different frequencies or frequency bands [12,13]. Although the adaptive filter weight coefficients are calculated in the frequency domain, the filter is implemented in the time domain. Work on the development of causal controllers to control impact noise such as radiated by punch presses has begun [14] and is in the process of further development. One area in which research is needed but where little is being done is in the application of adaptive feedback control to complex multi-channel acoustical systems. Some work in this area applied to impulsive or broadband disturbance control in enclosures was reported by Clark and Cole [15]. Other preliminary work has also been reported [16,17]. Another area in which research is currently being undertaken is in the application of genetic algorithms to active noise and vibration control systems. This will be discussed in detail in a future newsletter. 3.3 Error sensor optimisation for feedforward systems. This particular topic has attracted a considerable amount of attention at recent conferences. The main area of interest is the development of shaped PVDF film sensors which are bonded to the radiating surface and are shaped so that they produce an error signal proportional to the radiated sound field [18-23]. Clark and Fuller [24] investigated an alternative means of eliminating error microphones for control of sound radiation. This was done by using a model reference controller which involved driving accelerometer sensors on a vibrating structure to a non- zero reference level. This pre-determined reference level corresponds to a cost function which is originally chosen to minimise either the radiated sound power or the sound pressure at a number of points. An area of research which is still attracting interest is the optimisation of error sensor locations (if discrete microphones are used) so that the sound power radiated or some other global error criterion (e.g enclosure potential energy) is minimised [25,26]. Snyder and Hansen [25] showed that the optimal microphone locations for minimising radiated sound power are at the locations of greatest difference between the primary sound field and the calculated controlled sound field, assuming that the minimum radiated power cost function is accurately measured by the error sensors. Chou [27] showed that in an ideal non-redundant system the number of error sensors should be equal to the number of control sources. Another aspect of current error sensor research involves the investigation of the use of the use of sound intensity rather than sound pressure, thus allowing error sensors to be located in the near field of a sound source and at the same time provide a signal proportional to the energy radiated to the far field [28]. Other work involving the definition of a suitable error cost function for a specific situation has been reported by Fuller and Toffin [29]. In general, care must be taken to ensure that the error cost function minimises the quantity of interest if it is not feasible to measure the quantity directly. Fuller and Toffin [29] showed how the pressure of an external disturbance limited the effectiveness of power transmission through vibration isolators as a cost function for minimising the sound radiated by the isolated structure. 3.4 Control Sources Widespread application of active noise control will depend on the ability of transducer manufacturers to construct robust transducers capable of withstanding harsh environments. Work in this area is intensifying but for commercial reasons it is published only when products are ready to be marketed. Loudspeakers are now available which have been especially designed to the rigorous specifications and continuous operating requirements of active noise control systems (including corrosive environments). However, much remains to be done especially in increasing the source output powers and source durabilities. For sound radiated by vibrating surfaces, the use of alternative control sources to modify the surface vibration are being investigated. These alternative sources include piezoelectric crystal wafers, piezoelectric crystal stacks and magnetostructive transducers. Also being investigated as a sound radiation control source is adaptive acoustic foam [30] which is made by embedding a PVDF film actuator in a layer of acoustic foam and bonding it to the surface from which radiation or reflection is to be controlled. The optimisation of the locations of multiple control sources to minimise a radiated sound field is a problem of considerable complexity which has only recently begun to be addressed. See Wang et. al. [31], Ruckman and Fuller [32,33], Clark and Fuller [34]. This work has essentially involved the use of linear quadratic optimisation theory and in the case of Ruckman and Fuller's work, a subset of all possible actuator locations is selected based on practical considerations and an exhaustive search of this set is undertaken. More recently, genetic algorithms have been developed and adapted to the problem of optimising control source locations [35-38]. This work is in the process of being extended so that it is not necessary to use an array of discrete control source locations [39]. An alternative control source location optimisation process which is more complex to implement has been reported by Heck and Nagshineh [26]. 4.0 CONCLUSIONS Although the principles of active noise control are straightforward, the practical implementation of workable systems is fraught with difficulty. To develop and implement a successful system to solve a noise problem it is necessary to have a detailed understanding of the acoustic field to be controlled as well as a method of choosing the optimal values of various parameters to ensure an effective and stable result. This understanding is an essential part of appreciating the physical limitations of active noise control which exist independently of the electronic signal processing hardware and software. The need to develop this understanding is the reason for the continuing research into the physics of various acoustic problems. The desire to make systems more effective and practical and lower in hardware cost is the reason for continuing research into new controller hardware and software architectures and algorithms, and for continuing research into more robust, more efficient, easier to install and less expensive control sources and error sensors. All of this research will continue into the foreseeable future as faster electronic processing and better algorithms increase the complexity of problems which will be amenable to active control. REFERENCES 1. Lueg, P. (1933). Process of silencing sound oscillations. German Patent DRP No. 655,508. 2. Lueg, P. (1936). Process of silencing sound oscillations. U.S. Patent No. 2,043,416. 3. Olson, H.F. (1953). Electronic sound absorber. Journal of the Acoustical Society of America, 25, 1130-1136. 4. Olson, H.F. (1956). Electronic control of noise, vibration and reverberation. Journal of the Acoustical Society of America, 28, 966-972. 5. Kuo, S.M., Nadeski, M., Horner, T., Chyan, J. and Panahi, I. (1994). Fixed-point DSP implementation of active noise control systems. Proceedings of Noise Con '94, Institute of Noise Control Engineering, pp 337-342. 6. Minguez, A. and Receuro, M. (1994). Improvement of the estimate of the speaker-error microphone transfer function in an active noise controller. Proceedings of Noise Con '94, pp. 297- 302. 7. Wangler, C. and Hansen, C.H. (1994). Genetic algorithm adaptation of non-linear filter structures for active sound and vibration control. Proceedings of IEEE, ICASSP, Conference on Acoustics Speech and Signal Processing, April 18-22, pp. III505- III508. 8. Mackenzie, N.C. and Hansen, C.H. (1991). The use of an alternative adaptive algorithm with a lattice structured filter for a multi-channel active noise or vibration control system. Proceedings of Internoise '91, pp. 177-180. 9. Kewley, D. and Clark, R.L. (1994). Feedforward control with the higher harmonic time-averaged gradient (H-TAG) descent algorithm. Journal of the Acoustical Society of America 94, (3) Pt 2, 1816. 10. Guicking, D. (1994). A broadband active noise control system using a fast RLS algorithm. Proceedings of the Third International Congress on Air and Structure-borne Sound and Vibration, pp. 1361- 1368. 11. Snyder, S.D. (1994). Active control using IIR filters - a second look. Proceedings of ICASSP, Conference on Acoustics Speech and Signal Processing, IEEE, pp II241-II244. 12. Reichard, K.M. and Swanson, D.C. (1993). Frequency domain implementation of the filtered-x algorithm with on-line system identification. Proceedings of the 2nd Conference on Recent Advances in Active Control of Noise and Vibration, Virginia Tech, pp. 562-573. 13. Shen, Q. (1994). A frequency domain multichannel optimal adaptive algorithm for active control of sound and vibration. Proceedings of Noise Con '94, pp. 321-324. 14. Snyder, S.D. and Tanaka, N. (1992). The active control of impact noise. Proceedings of the Japanese Society of Mechanical Engineers Dynamics and Design Conference, Niseko, Japan, pp.213- 218. 15. Clark, R.L. and Cole, D.G. (1994). Active damping of enclosed sound fields through direct rate feedback control. Journal of the Acoustical Society of America 95 (5) pt 2, 2989. 16. Kuo, S.M. and Vijayan, D. (1993). Feedback active noise control systems. Proceedings of Noise Con '94, pp. 132-141. 17. Kuo, S.M. and Vijayon, D. (1994). Adaptive feedback active noise control. Proceedings of Noise Con '94, pp. 473-478. 18. Snyder, S.D., Hansen, C.H. and Tanaka, N. (1993). Shaped vibration sensors for feedforward control of structural radiation. Proceedings of the Second Conference on Recent Advances in Active Control of Sound and Vibration, Virginia Tech., pp. 177-188. 19. Clark, R.L. and Burke, S.E. (1994). Practical considerations for designing shaped modal sensors from polyvinylidone flouride. Journal of the Acoustical Society of America 95 (5) pt 2, 2990. 20. Naghshineh, K. and Koopman, G.H. (1992). A design method for achieving weak radiator structures using active vibration control. Journal of the Acoustical Society of America 92, 856-870. 21. Fuller, C.R. and Burdisso, R.A. (1991). A wavenumber approach to the active control of sound and vibration. Journal of Sound and Vibration 148, 355-360. 22. Maillard, J.P. and Fuller, C.R. (1993). Active structural acoustic control with broadband disturbances and realtime structural wavenumber sensing. Journal of the Acoustical Society of America 94, 1816. 23. Sommerfeld, S.D. and Scott, B.L. (1994). Estimating acoustic radiation using wavenumber sensors. Proceedings of NoiseCon '94, pp. 279-284. 24. Clark, R.L. and Fuller, C.R. (1992). A model reference approach for implementing active structural acoustic control. Journal of the Acoustical Society of America 92, 1534-1544. 25. Snyder, S.D. and Hansen, C.H. (1991). Using multiple regression to optimise active noise control system design. Journal of Sound and Vibration 148, 537-542. 26. Heck, L.P. and Nagshineh, K. (1994). Large-scale broadband actuator selection in active noise control. Proceedings of Noise Con '94, pp. 291-296. 27. Chou, K.C. (1994). Selecting numbers of transducers, filter lengths for neutralized-feedforward active noise control. Proceedings of Noise Con '94, pp. 285-290. 28. Swanson, D.C., Gentry, C, Hayek, S.I. and Sommerfeldt, S.D. (1994). Adaptive control of bending wave intensity in a finite beam. Journal of the Acoustical Society of America 94 (3) Pt 2, 1817. 29. Fuller, C.R. and Toffin, E. (1994). Passive-active isolator control of sound radiation from a raft-cylinder system. Journal of the Acoustical Society of America 95 (5) pt 2, 2987. 30. Fuller, C.R., Bronzel, M.J., Gentry, C.A. and Whittington, D.E. (1994). Control of sound radiation/reflection with adaptive foams. Proceedings of Noise Con '94, pp. 429-436. 31. Wang, B., Burdisso, R. and Fuller, C.R. (1991). Optimal placement of piezoelectric actuators for active control of sound radiation from elastic plates. Proceedings of Noise Con '91, Institute of Noise Control Engineering. pp. 267-274. 32. Ruckman, C.E. and Fuller, C.R. (1993). Optimising actuator locations in feedforward active control systems using sub-set selection. Proceedings of 2nd conference on Recent Advances in Active Control of Sound and Vibration; Virginia Tech., pp. S122- S133. 33. Ruckman, C.E. and Fuller, C.R. (1994). Detecting and analysing collinearity in simulations of feedforward active noise control. 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Optimal placement of actuators in actively controlled structures using genetic algorithms. AIAA Journal, 29, 942-943. 39. Simpson. M. and Hansen, C.H. (1994). Genetic algorithm optimisation of multiple actuator locations for active control of sound transmission into a cylindrical enclosure. To be published. ***************************************************************** *** END of ISSUE 2